Why would it be necessary to have an engineer on call 24/7 ? If you do your risk calculations and an outage of 12 hours is acceptable in the expected frequency you just let the engineer have a nice evening and night an deal with the outage in the morning.
If outages are only to be expected once a year an you can tolerate 48 hours of outage you don't need any on call engineer. Most outages are cause by changes. You can test those and plan putting these in production and have elevated monitoring after this to catch problems early.
Only problem remaining is hardware outages. And those are very rare as long as you do decent lifecycle maintenance.
As others said before. Not everyone is Google or Amazon and needs 99,999% uptime.
